Conventional converter active devices in a converter which is connected to a three-phase network maybe actuated by drive signals which are synchronized to the phase angle of the network voltage system using a phase locked loop. This conventional phase locked loop is based on the fact that a three-phase system can be represented by orthogonal components of a vector in a suitable coordinate system. In this case, actual values of the three-phase system are transformed by means of a vector rotator into the components of the voltage vector in a rotating coordinate system. The transformation elements of this vector transformation are obtained by supplying an angle regulator with the mean angle error between the transformed vector and a direction which is permanently defined in the rotating coordinate system. The output signal from this angle regulator governs the frequency of the rotating coordinate system and controls the frequency of oscillators whose output signals on the one hand supply the angular functions (which are required as transformation elements) of the transformation angle, while on the other hand also supplying the phase angle of the reference voltages which are synchronized to the network alternating voltage.
For stable operation of a three-phase network in the case of a self-commutated network converter of a converter with a variable voltage link, the earth currents i.sub.u, i.sub.v, i.sub.w, taken from the network must be regulated. Generally, no reactive volt-amperes, or predetermined reactive volt-amperes should be taken from the network, so that the phase angle of the currents with respect to the network voltages must be permanently predetermined. The amplitude of the phase currents, on the other hand, is a function of the real power and reactive volt-amperes to be transmitted. It is thus necessary for the drive signals for the converter active devices to be synchronized to the network voltages. This can be done by the vector filter described above although, however, this results in a large number of components, and costs associated with them. Furthermore, owing to the finite dynamics of the angle control loop, network fluctuations result in an error angle between the phase voltages and reference voltages.
The network feedback effects on the three-phase network caused by converters are also problematic. Whenever energy is fed from the three-phase network to a variable voltage link, or is recovered from the variable voltage link to the three-phase network, using phase currents in the form of blocks, harmonics (5th, 7th, 9th, etc. harmonics of the network frequency) occur. However, such network feedback effects are undesirable in terms of electromagnetic compatibility.
In order to minimize such network feedback effects, a converter is known which uses a phase regulator to form an analog angle signal which is synchronized to a linked voltage, in order to address function memories in order to form digital angular function values. A DC voltage regulator supplies the nominal value for the amplitude of the system of alternating currents to be taken from the alternating voltage network. By multiplying the digital angular function values by the nominal amplitude, digital/analog converters supply a system of phase current nominal values, which system is compared with corresponding current actual values. The comparison results are used to drive lower-level current regulators, whose output signals supply drive signals for the active devices in the converter (e.g. transistors). However, such a control concept has the disadvantage that it is not robust if one phase of the three-phase network fails since, in such an event, the control system goes into a positive feedback mode, which results in the transistors that are used as converter active devices being destroyed.